The present invention relates to a method for removing hydrogen sulfide compounds from liquid sulfur, by passing a finely divided gas through liquid sulfur.
It is known from Journal of Physical Chemistry, Vol. 70, no.1, 234-238 that hydrogen sulfide in liquid sulfur is dissolved in the form of polysulfides, designated as H.sub.2 S.sub.x where x is an integer of at least 5, and in the form of physically dissolved H.sub.2 S. Through decomposition of polysulfides, hydrogen sulfide is liberated. In the present text the terms hydrogen sulfide and hydrogen sulfide compounds will be used in the sense of both H.sub.2 S and H.sub.2 S.sub.x, unless specified otherwise.
Sulfur produced in sulfur recovery plants contains on average 300 to 400 ppm by weight of hydrogen sulfide and polysulfides. During storage, transport or further application, the release of dissolved hydrogen sulfide can give rise to dangerous situations, for instance where people are stupefied by the very toxic H.sub.2 S, sometimes with a fatal result (600 ppmv is already lethal to humans), and the risk of explosion due to hydrogen sulfide released in the headspace of, for instance, storage tanks (the lower explosion limit is about 3.5% by volume of hydrogen sulfide in air). Also, problems of stench due to hydrogen sulfide can be a great nuisance. In installations that produce or process sulfur it is therefore required that the sulfur produced be degassed to remove hydrogen sulfide and polysulfides to values below 10 ppm by weight.
Dissolved hydrogen sulfide is easy to remove from liquid sulfur, for instance by stirring, spraying, pumping or by passing gas or air through it. It is considerably more difficult to remove the polysulfides. Polysulfides first have to be decomposed according to the reaction EQU H.sub.2 S.sub.x .fwdarw.H.sub.2 S+(x-1) S
before the hydrogen sulfide then formed can be removed from the liquid sulfur by degassing EQU H.sub.2 S (dissolved).fwdarw.H.sub.2 S (gas)
The decomposition of polysulfides can be promoted through addition of nitrogenous compounds such as ammonia, ammonium salts, organic nitrogen compounds (such as alkyl amines, alkanol amines or aromatic nitrogen compounds) or urea. These nitrogen compounds function as a catalyst and thus shorten the decomposition time and hence the time required for degassing.
Societe Nationale des Petroles d' Aquitaine developed a sulfur degassing process whereby the sulfur is pumped round and sprayed, with ammonia being added as catalyst (French patent no. 1,435,788). SNPA, later known as SNEA (Societe Nationale Elf Aquitaine), improved the process from a non-continuous to a continuous process in which the sulfur is circulated over two compartments and sprayed. Here too, ammonia is added as catalyst. These process variants are described in Hydrocarbon Processing October 1992 (pp. 85-89). SNEA has improved the process once more through the use of a liquid catalyst. This process is known under the name of Aquisulf. In this process too, the sulfur is circulated and sprayed. The Aquisulf process is described in Oil and Gas Journal Jul. 17, 1989, pp. 65-69.
Exxon developed a sulfur degassing process by adding a liquid catalyst in the sulfur pit or tank. In the Exxon process the sulfur is not circulated or agitated in any other way. The process saves energy but proper degassing requires a residence time of 3 to 4 days. The process is described in CEP October 1985, pp. 42-44 and in Hydrocarbon Processing May 1981, pp. 102-103.
Texas Gulf has developed a sulfur degassing process in which liquid sulfur flows down a column over dishes and the sulfur is degassed countercurrently with air (U.S. Pat. Nos. 3,807,141 and 3,920,424).
Shell Internationale Research Maatschappij developed a sulfur degassing process that is described in Dutch patent 173,735.
This method consists of a single process step in which air or a mixture of an inert gas and oxygen is passed through liquid sulfur in the presence of a catalyst, typically a nitrogen compound, in finely divided condition and thereafter the liquid sulfur and the used gas are separated from each other.
A comparable method is described in DD-A 292,635. According to this method, the treated sulfur, prior to the further processing, is subjected to a supplementary post-gassing. However, such a post-gassing has no effect or substantially no effect on the reduction of the sulfide content in the liquid sulfur.
Procor developed a sulfur degassing process known under the name of "HySpec", in which a number of gas-liquid contact mixers are arranged in series. A catalyst is added to the contact mixers and finally in the last mixing stage the sulfur is stripped of the added catalyst by passing air through it. Such a gas-liquid contact mixer consists of a mixer driven by an electric motor, which circulates the sulfur with drawn-in air over a perforated cylinder. This process was presented at the Sulphur '94 conference at Tampa Fla., Nov. 6-9, 1994 (see also WO-A 95/06616). A disadvantage of this method is the use of moving parts such as an agitator which comes into contact with the liquid sulfur. In systems with liquid sulfur, there is a great chance that moving parts will jam.
As mentioned earlier, sulfur degassing plants can be made of smaller size owing to the use of a catalyst. However, the addition of catalysts has many associated drawbacks with regard to the decrease of the quality of the sulfur. It is also known that problems of clogging readily occur as a result of these catalysts due to the formation of salts, such as ammonium sulfate. Many complaints have been heard from buyers of sulfur in the production of sulfuric acid. Some large sulfur buyers accordingly require that sulfur contain no catalyst.
Also well-known are problems of corrosion caused by the presence of salts. Many companies have had to adapt their sulfur degassing plants or have opted from the outset for a process in which no catalyst is used. A major disadvantage is that such a process requires a much longer degassing time, which entails higher investments and involves a higher energy consumption.